Proton user_settings.py ignored in Steam Decks 'Gaming Mode'

Your system information

Hardware: Steam Deck

SteamOS:

BUILD_ID=20221005.1
VERSION_ID=3.3.2

Steam: Dec 5 2022, at 23:38:29

Steam API: v020

Steam package version: 1670284449

Please describe your issue in as much detail as possible:

I wanted to configure Proton via the user_settings.py, which seems to work when the game is started with Steam from the Desktop Mode. However, those settings seem to be ignored when the game is started from the Steam Decks 'Gaming Mode'.

Steps for reproducing this issue:

Install a Unity Windows game and Proton 7.0 on a Steam Deck
Install Bepinex 5 (Unity modding tool) for the game, it needs to override winhttp.dll to work.
Set the following config: /home/deck/.local/share/Steam/steamapps/common/Proton 7.0/user_settings.py:
```
user_settings = {
    "WINEDLLOVERRIDES": "winhttp=n,b",
}
```
Start game from Steam in Desktop mode. A log file in the game directory [code]BepInEx/LogOutput.log[/code] should be created, meaning the modding tool was successfully loaded.
Delete the log file and change to 'Gaming Mode'
Start the same game from there

Expected:

The log file is created as well, meaning the modding tool was loaded and the game is started the same way in the 'Gaming Mode' as it does in the 'Desktop Mode'

Result:

No log file is created, meaning the modding tool was not loaded, which means the [code]user_settings.py[/code] was not loaded.

Possible workarounds and their downsides:

Use a wrapper script/program to set options and start the game (local to each game)
Set game launch options containing the environment variables (local to each game)
Modify the proton python script (modifies code, that should not be modified)

Other infos

I also posted this on the steam forums: https://steamcommunity.com/app/1675200/discussions/1/3716063566028107561/

ValveSoftware / Proton